Controlled welding technique for a silver container

ABSTRACT

A technique for welding which controls the depth of penetration of a nickel wire into the bottom of a silver capacitor can having a thickness range of approximately 0.010 to 0.020 inch. The technique limits and advantageously controls the formation of columnar structure in the weld area. This control is achieved by forming serrations or the like on the bottom of the can prior to the attachment of the nickel wire to be welded thereon, so that a proper heat balance is created thereon that permits a strong bond between the nickel wire and the silver can to be formed.

United States Patent Zeppieri 1 Dec. 19, 1972- [541 CONTROLLED WELDINGTECHNIQUE 3,015,758 1/1962 Hildebrandtet al ..317/230 FOR A SILVERCONTAINER 3,243,316 3/1966 O'Nan ...3l7/230 3,534,230 10/1970 Krasienko..3l7/230 [72] Inventor: Dominick John Zeppieri, North v Adams Mass-Primary Examiner-James D. Kallam [73] Assignee: Sprague ElectricCompany, North AttorneY -Anhur Connolly et aL Adams, Mass. [57] ABSTRACT[22] Filed: Sept. 28, 1971 4 A technique for welding which controls thedepth of PP 184,398 penetration of a nickel wire into the bottom of asilver capacitor can having a thickness range of approxi- [52] us. Cl...317/230, 29/570 317/242 0-010 0-020 inch- The technique limits and 511mu. .Jim 9/08 advamagmsly the [58] Field 0 Search 317/230 231 232 233structure in the weld area. This control is achieved by 1 formingserrations or the like on the bottom of the can prior to the attachmentof the nickel wire to be welded thereon, so that a proper heat balanceis [56] Reierences Cited created thereon that permits a strong bondbetween ITE AT S PATENTS the nickel wire and the silver can to beformed.

5/ 1958 Boae .317/230 6 Claims, 3 Drawing Figures CONTROLLED WELDINGTECHNIQUE FOR A SILVER CONTAINER BACKGROUND OF THE INVENTION Columnarstructure is a coarse structure of parallel 1 columns of grains whichare caused by highly directional solidification resulting fromsharpthermal gradients. Ordinarily this is not a detriment to manywelding operations, however, it is a detriment to tantalum wet capacitorsystems that use highly acidic electrolytes. For example, a system using40 percent sulfuric acid as an electrolyte has leakage problems becausethe acid attacks any columnar structure present.

When two metals such as silver and nickel, being so completely differentin thermal characteristics, are welded together by conventionaltechniques, an improper heat balance results. An improper heat balanceis a condition in which one of the two metals to be welded melts at agreater rate than the other causing columnar structure and poor welds.Specifically, upon welding the silver becomes generally molten andcauses some nickel to be dispersed therethrough all the way to the innersurface of a silver capacitor can. This phenomenon makes a silvercontainer having a nickel lead wire welded thereto susceptible to acorrosive attack of the nickel contaminated silver by the acidelectrolyte. Tantalum wet capacitors utilize this construction, and areused in high precision and high priced equipment where leaking sulfuricacid can have disastrous effects thereon.

Prior art attempts to overcome this vexing problem have beenunsuccessful and have included conventional resistance welding andpercussion welding. However, both of these welding processes producecolumnar structure and neither process alone offers a proper solution tothe problem of leakage.

Accordingly, it is an object of the present invention to provide acontrollable method of welding thermally dissimilar metals, such assilver and nickel.

It is another object of this invention to correct the improper heatbalance so as to provide a grain structure in the weld area that isdevoid of columnar structure.

It is a further object of the present invention to provide a method ofwelding a nickel lead wire to -a silver container in such a manner thatthe container will not leak when a highly acidic electrolyte is placedtherein.

SUMMARY OF THE INVENTION Serrations are formed on the bottom exterior ofa silver container at the point where a nickel wire is to be weldedthereon. These serrations permit the achievement of a proper heatbalance when two such thermally different metals are welded together byconventional welding processes. The serrations on the silver cause abuild up of a higher than normal current density thereon, and this inturn contributes to a proper heat balance. The heat produced from thewelding operation is concentrated on the projections formed by theserrations of this invention. For a median can-base thickness ofapproximately 13 mils, satisfactory results are obtained when there isapproximately 5.0 mils of unaffected silver between the electrolyte andthe nickel lead wire.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of the serratedsurface of a silver container;

FIG. 2 is a cross-sectional view of the silver can 0 showing theserrations thereon; and

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A silver capacitor canhas serrations or projections impressed or embossed on the closed endthereof by any conventional technique known to one skilled in the art,such as by a punch and die. These serrations limit and control theformation of columnar structure when a silver capacitor can has athermally different metal lead wire welded thereto.

In FIG. 1 there is shown the bottom exterior surface of a silvercapacitor can 10 having serrations or projections 11 formed thereon inthe form of concentric rings. These ringed serrations can be formed byany conventional embossing or impressing technique known to one skilledin the art. FIG. 2 shows the approximate depth of the serrations 11 intothe can 10. The depth should be approximately 0.004 i 0.001 inch intothe bottom of the can, for cans having a surface that is at least 0.010inch thick. Ifthere is no unaffected silver area between the interiorsurface of the can and the nickel lead, leakage problems are confronted.

FIG. 2 also shows typical dimensions of serrations made in a capacitorcan. The distance between serrations herein is 0.0029 inch, while theangle measured by the slope of adjacent serrations is approximatelyHowever, this is only typical and should not be' considered mandatoryrequirements. The serrations should be made in such a manner that thediameter of the nickel wire should substantially cover the serrationswhen welded to the can, and the angle of slope described by adjacentserrations should permit a plurality of these serrations to be formed inan area small enough to be substantially covered by the diameter of thewire lead welded thereon.

The serrations need not necessarily be in the form of concentric rings.Alternatively, cross-hatched serrations may be used for the purposes ofthis invention. These serrations may be formed as by a punch and die orother technique known to one skilled in the art. Again, the penetrationof the serration should be approximately 0.004 t 0.001 inch on any sizecan, as this depth of serration is sufficient to concentrate enough heatin the serrated area so as to produce a proper heat balance between thetwo metals to be joined herein.

An opposed electrode welding process is used to attach the lead wires tothe silver cans. In the process, the can is connected physically andelectrically to a first electrode of a certain polarity, and a lead wirehaving contact to a second electrode of opposed polarity is brought intocontact briefly with the silver can. Upon touching the serrated portionof the can the nickel wire is welded instantaneously into place,advantageously penetrating only to the depth of the serrations. Thisprocess takes place rapidly because the serrations in the can cause thebuild-up thereon of -a high current density, and therefore a highlyconcentrated hot area.

It has been discovered that when such serrations are formed on thecapacitor cans prior to the welding operation, the heat generated duringthe operation is concentrated at the projections formed thereby. Thisprovides the needed heat balance which allows the formation of a strongconsistent weld bond, and at the same time limits and controls theformation of columnar structure and the depth of penetration of the leadinto the can. Ordinarily, silver, being a good conductor, does not buildup any heat at any one point, however, the serrations of this inventioncause current used in the welding operation to be forced through theprojected areas, and a higher current density is localized at thesepoints producing a proper heat balance between the two metals to bewelded together.

FIG. 3 shows a capacitor can having a nickel lead wire 12 weldedthereto, completely covering the serrated configuration formed for thepurposes of this invention. Microscopic observation will, however, showthat the outer perimeter of serrations 13 t hat had been made on the canstill remain under the welded lead wire, while the inner serrations havefused with the lead to form a strong bond therewith. Cans formed thuslywill not leak when a highly acidic electrolytic solution is usedtherein.

The above-described specific embodiments of the invention have been setforth for the purposes of illustration. It will be apparent to thoseskilled in the art that various modifications may be made in thegeometrical design of the serrations or projections formed withoutdeparting from the principles of this invention as pointed out anddisclosed herein. For that reason, it is not intended that the inventionshould be limited other than by the scope of the appended claims.

What is claimed is: 1. A method of welding a metal lead wire to a silvercontainer, where said 'metal lead wire has thermal 5 characteristicsthat differ from silver, comprising:

forming serrations on the bottom exterior surface of a silver container,said serrations being of such a size as to be completely covered by ametal wire welded thereto; placing said serrated container in electricalcontact with a first electrode of a certain polarity; afiixing a metallead wire to a second electrode of a polarity opposing that of saidfirst electrode, said metal lead wire being of a metal that has thermalcharacteristics that differ from silver; and contacting said metal leadwire to said container at the point of said serrations causing a highcurrent density to be localized at said serrations and a welded bond tobe formed thereon.

2. The method of claim 1 wherein said serrations are formed to have adepth of penetration in the silver container of 0.004 t 0.001 inch.

3. The method of claim 2 wherein said metal lead wire is nickel, andsaid serrations are formed as concentric rings. a

4. The method of claim 2 wherein said metal lead wire is nickel, andsaid serrations are formed as a crosshatched impression on the silvercan.

5. An electrolytic capacitor container comprising: a silver containerhaving an open end and a closed end; a metal lead wire attached to saidsilver container at the closed end, said, metal lead wire is of a metalhaving thermal characteristics that differ from silver, and said silvercontainer having serrations on said closed end 3 5 under the outerperimeter of said metal lead wire.

6. The container of claim 5 wherein said metal lead wire is nickel, andsaid serrations are ring shaped.

2. The method of claim 1 wherein said serrations are formed to have adepth of penetration in the silver container of 0.004 + or - 0.001 inch.3. The method of claim 2 wherein said metal lead wire is nickel, andsaid serrations are formed as concentric rings.
 4. The method of claim 2wherein said metal lead wire is nickel, and said serrations are formedas a cross-hatched impression on the silver can.
 5. An electrolyticcapacitor container comprising: a silver container having an open endand a closed end; a metal lead wire attached to said silver container atthe closed end, said metal lead wire is of a metal having thermalcharacteristics that differ from silver, and said silver containerhaving serrations on said closed end under the outer perimeter of saidmetal lead wire.
 6. The container of claim 5 wherein said metal leadwire is nickel, and said serrations are ring shaped.